In order to determine or assess the ability of a specific coronary vessel to supply blood to the heart muscle, i.e. the myocardium, there is known a method by which the intracoronary pressure distally of a stricture in combination with the proximal pressure is measured. The method is a determination of the so called Fractional Flow Reserve (see “Fractional Flow Reserve”, Circulation, Vol. 92, No. 11, Dec. 1, 1995, by Nico H. j. Pijls et al.). Briefly FFRmyo is defined as the ratio between the pressure distally of a stricture and the pressure proximal of a stricture, i.e. FFRmyo=Pdist/Pprox. The distal pressure is measured in the vessel using a micro-pressure transducer, and the proximal pressure is the arterial pressure.
A limitation in measuring only the blood pressure and the pressure gradient, alternatively the Fractional Flow Reserve, is that there is no control of the flow in the coronary vessel. As an example, a vessel having a significant stricture would not yield any pressure drop if the myocardium is defective and has no ability to receive blood. The diagnosis will incorrectly show that the coronary vessel is healthy, when instead the conclusion should have been that the myocardium and possibly the coronary vessel are ill.
A diagnosis method for diagnosing small vessel disease is performed as follows:
The Fractional Flow Reserve is determined. If the FFR is <0.75 the coronary vessel should be treated.
If FFR is >0.75 there are two possibilities:
a) either the patient is healthy with respect to the actual coronary vessel (the most plausible), or
b) there is a low blood flow distally of the distal pressure measurement due to either an additional stricture or a sickly myocardium.
In order to investigate whether alternative b) is at hand, it is desirable to obtain knowledge regarding the health status of the myocardium, by measuring Coronary Flow Reserve (CFR), or in the alternative the Coronary Velocity Reserve (CVR). The idea is to determine by how many times a patient is able to increase his/her blood flow during work. A healthy patient should be able to increase the blood flow by 2.5-5 times, depending on the patient's age. Work is simulated by the addition of a so called vaso dilating pharmaceutical/medicament, e.g. Adenosine, Papaverin or the like. This medicament dilates the capillaries which increases the blood flow. The same medicament is used for determining FFR.
CFV is defined as                     CFV        =                              Q            work                    /                      Q            rest                                                  =                              Q                          during              ⁢                                                           ⁢              vaso              ⁢                                                           ⁢              dilation                                /                      Q            rest                              (Q is the flow).
This being a ratio and assuming that the cross sectional area is constant during one velocity measurement, it will suffice to measure the velocity.
CFR is defined asCFR=Qwork/Qrest=[Vwork*K]/[Vrest*K]=Vwork/Vrest
Since the desired parameter is a flow increase, it will be sufficient to obtain it as a relative quantityCFR=[K*Vwork]/[K*Vrest]wherein K is a constant.
Researchers have devised methods where the pressure and flow velocity in the coronary vessel are measured, the results being presented as so called “pressure-velocity loops” (di Mario). Thereby it becomes possible to distinguish patients suffering from the so called “small vessel disease” from others. In patients with “small vessel disease” the pressure gradient, corresponding to a low FFR, and the velocity of flow will be low, whereas healthy patients will have a low pressure gradient, corresponding to a high FFR, and a high flow.
In some investigations the applicant's system for pressure measurements in vivo, Pressure Guide™ (Radi Medical Systems) and the flow sensor sold under the trade name Flowmap™ (Cardiometrics) have been tested.
It is a great drawback to have to introduce two sensors into the coronary vessel, compared to a situation where both sensors are mounted on a “guide wire”. Thus, it has been suggested to provide a guide wire with two sensors, but this presents several technical problems with the integration of two sensors in a thin guide wire.